rados: do not dump usage on invalid command

I hate this; it makes it impossible to see that there was an error message.

We made this same change a while back with rbd.

Signed-off-by: Sage Weil <sage@inktank.com>
rbd: be helpful with invalid command

$ rbd asdf
rbd: error parsing command 'asdf'; -h or --help for usage

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #673 from liewegas/wip-usage

make rbd, rados bad command errors more friendly